The process of thermal melt transfer (also known as thermal mass transfer or wax transfer) is well known in the art. In this technology, a dye or a pigment is dispersed in a binder, which has a low melting point. The dispersion is coated as a coloured layer onto an elongate strip or ribbon of a heat-resistant substrate, typically polyethylene terephthalate film, and is used to print onto plain paper or other receiver media. In the printing process, the ribbon is in contact with the receiver medium, while moving through the nip between a thermal head and a roller. Usually, the thermal head extends across the entire width of the ribbon and media, and consists of a line of individually addressable electrical heating elements. The elements are activated so as to transfer the coloured layer from the ribbon to the receiver medium, in order to print, for example, text, a bar code, or even a half-tone image. The nature of the printing process is essentially binary—the heated area of the coloured layer transfers completely, and this is the reason that any images printed can only be half tone, rather than continuous tone as in a photograph.
Multicolour images can be printed by using a ribbon carrying a plurality of similar sets of different coloured layers, each set comprising a panel of the subtractive primary colours (yellow, magenta and cyan) with an optional black panel, with the panels being in the form of discrete stripes extending transverse to the length of the ribbon, and arranged in a repeated sequence along the length of the ribbon. Such images are still subject to the binary nature of the melt transfer process and are coarse in nature.
The process of thermal dye transfer is also well known. The ribbon used is very similar in appearance to the coloured ribbon used in melt transfer, but the composition of the panels is different. Whereas dyes or pigments may be used for melt transfer, pigments cannot be used for dye transfer, as it is essential to use colorants that are capable of dissolving in, and migrating through, the polymers that make up the coatings on the ribbon and on the receiver media. The dyes chosen are typically soluble in organic solvents and are typically coated onto the ribbon in a polymeric binder. The receiver medium normally needs a smooth polymeric surface in order to be in intimate contact with the ribbon during the printing process and to receive the dyes. Only the dyes transfer during printing, and the polymeric binder remains in place on the ribbon.
The printing process is similar to that described above for melt transfer, but because the dye is transferred by a molecular diffusion process, the amount transferred at each point is determined by the amount of heat applied by the thermal head. By varying the amount of heat applied at each point during printing, it is thus possible to achieve a continuous tone image, which is of much higher quality than the half tone images achievable using melt transfer. Indeed, photographic quality images are available by this printing process.
A printer is normally designed to take an electronic image, such as might be displayed on a cathode ray tube (CRT) and to reproduce it faithfully as a printed image. In order to do this, the red, green and blue (RGB) additive colours used must be converted to cyan, magenta and yellow (CMY) subtractive primary colours for printing. This is essentially an inversion process, as cyan absorbs red light, magenta absorbs green light and yellow absorbs blue light.
Fluorescent materials may also be transferred thermally. For example the melt transfer of fluorescent pigments is described in JP59-054598. Fluorescent dyes have also been transferred, for example as described in EP374835A1.
JP2000141863 describes the use of multicoloured mass transfer of fluorescent pigments in order to build up a full colour image onto a security card. Because of the binary nature of the mass transfer process, the quality of such an image is necessarily poor.